ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 201 PM EST SUN DEC 03 2017 VALID 12Z THU DEC 07 2017 - 12Z MON DEC 11 2017 ...STORMY PATTERN EXPECTED FOR SOUTHERN AND PORTIONS OF SOUTHEASTERN ALASKA... A BLOCKING FLOW PATTERN WILL KEEP THE WEATHER RATHER UNCHANGED ACROSS ALASKA THROUGH THE MEDIUM RANGE. AN ANOMALOUS UPPER RIDGE IS EXPECTED TO BUILD ACROSS WESTERN NORTH AMERICA DURING THE SHORT RANGE, WITH 500 HPA HEIGHT ANOMALIES EXCEEDING +3 STANDARD DEVIATIONS POSSIBLE BY DAY 4 (THU) ACROSS NORTHERN BRITISH COLUMBIA. UPSTREAM OF THE RIDGE, PERSISTENT NEGATIVE HEIGHT ANOMALIES ARE EXPECTED ACROSS THE NORTH/CENTRAL PACIFIC WITH AN ACTIVE TRAIN OF SHORTWAVES ORIGINATING ACROSS EAST ASIA AND QUICKLY AMPLIFYING AS THEY ENTER THE LARGER SCALE TROUGH. WHILE CONFIDENCE IS ABOVE AVERAGE WITH RESPECT TO THIS LARGE SCALE FLOW PATTERN, THE DETAILS ARE MUCH MURKIER. MODEL SPREAD SURROUNDING THE SMALLER SCALE WAVES IS VERY HIGH, ESPECIALLY BY LATE IN THE WEEK. DETERMINISTIC SOLUTIONS SEEM TO SHOW SOME LEVEL OF AGREEMENT THAT A RELATIVELY COMPACT AREA OF LOW PRESSURE WILL QUICKLY MOVE NORTHWARD INTO SOUTHERN ALASKA ON THU, ALTHOUGH ENSEMBLE SPREAD IS STILL SOMEWHAT HIGH AND MODEL RUN-TO-RUN TIMING DIFFERENCES REMAIN (E.G. THE 12Z GFS WAS MUCH QUICKER WITH MOVING THIS SYSTEM INLAND RELATIVE TO THE PREVIOUS CONSENSUS FROM OVERNIGHT). AFTER THU MODEL SPREAD QUICKLY INCREASES EVEN FURTHER, BUT THERE IS A GENERAL CONSENSUS (REPRESENTED BY THE ENSEMBLE MEANS) THAT AN AREA OF LOW PRESSURE SHOULD LINGER SOUTH OF THE ALASKA PENINSULA THROUGH DAYS 5-7. MULTIPLE SHORTWAVES WILL LIKELY PASS OVER THE REGION DURING THIS TIME FRAME, WITH THE POTENTIAL FOR SMALL-SCALE LOWS TO MOVE NEAR THE COAST OR EVEN ONSHORE, BUT FORECAST CONFIDENCE FOR SUCH FEATURES IS VERY LOW AT THIS POINT. ANOTHER MORE VIGOROUS SHORTWAVE LOOKS TO MOVE INTO THE BERING SEA BY LATE SAT. WHILE MODELS GENERALLY AGREE ON THE EXISTENCE OF THIS FEATURE AND THE ASSOCIATED SURFACE CYCLONE, THEY DIFFER SUBSTANTIALLY ON THE STRUCTURE OF THE SYSTEM AS WELL AS THE TIMING. GIVEN THESE CONSIDERATIONS, THE WPC FORECAST WAS INITIALLY BASED ON A MULTI-MODEL DETERMINISTIC BLEND ON DAY 4, INCLUDING THE ECMWF/CMC/GFS. THE FORECAST WAS QUICKLY TRENDED TO MAJORITY ENSEMBLE (ECENS AND NAEFS) WEIGHTING FROM DAY 5 ONWARD, HOWEVER. DETERMINISTIC MODELS WERE REMOVED FROM THE FORECAST BLEND ENTIRELY BY DAY 8, GIVEN THE VERY LARGE SPREAD. EVEN THE ENSEMBLE MEANS SHOULD BE USED WITH MUCH CAUTION BY THAT TIME AS ENSEMBLE SCATTER PLOTS SHOW EXCEEDINGLY LARGE SPREAD AND LITTLE TO NO CLUSTERING OF SOLUTIONS. IN TERMS OF SENSIBLE WEATHER, THE IMPLICATIONS OF THIS PATTERN ARE A BIT MORE CLEAR THAN THE DETAILS OF THE MASS FIELDS. A STORMY PERIOD IS IN STORE FOR SOUTHERN AND SOUTHEASTERN ALASKA WITH PERSISTENT AND DEEP SOUTHERLY FLOW TRANSPORTING COPIOUS AMOUNTS OF PACIFIC MOISTURE NORTHWARD INTO THE REGION (PWAT ANOMALIES REACH +2 TO +3 STANDARD DEVIATIONS THROUGH MUCH OF THE PERIOD), WITH THE POTENTIAL FOR RELATIVELY HEAVY PRECIPITATION ACROSS MULTIPLE DAYS (PARTICULARLY ACROSS FAVORED/WINDWARD TERRAIN). SNOW SHOULD BE A BIT MORE SPARSE ACROSS CENTRAL/NORTHERN ALASKA, BUT SCATTERED SNOW SHOWERS ARE STILL EXPECTED ESPECIALLY ACROSS THE BROOKS RANGE. TEMPERATURES SHOULD GENERALLY BE NEAR OR SLIGHTLY ABOVE AVERAGE GIVEN THE SOUTHERLY FLOW AS WELL AS THE UPPER RIDGING NOSING INTO THE STATE. RYAN